Systems for mixing and dispensing singular and multicomponent materials are well known in the art. Such systems typically include pumping mechanisms for pumping and metering separate materials, such as a base material and an accelerator material, in a prescribed ratio to a mixing device that thoroughly mixes these materials together. The mixed composition then flows out of a dispensing nozzle directly to the surface or point of application where the composition is desired.
When a curable composition is desired, two or more suitable materials are mixed to interact with each other to create a flowable, curable composition which will set or harden to a non-flowable state. The time required for a curable composition to harden is referred to as the "cure" time and often is a short period of time. Such resulting curable compositions have been used, for instance, as adhesives, sealants and potting materials in a wide variety of industrial applications.
Production environments can impose limitations on how a dispensing device should operate. For example, in a production environment, it is desirable for the curable composition to cure as rapidly as possible so that subsequent production operations can be performed on the production item without having to wait a significant time for curing to occur.
Further, production requirements often include the need to dispense a precise amount of a properly constituted composition. A deviation in the actual ratio of the constituent materials dispensed may alter the strength, viscosity and/or other properties and attributes of the composition. Thus, a dispensing system should dispense the desired ratio and quantity of constituent materials as accurately as possible. In many cases, the desired ratio is expressed as a function of the weight or mass of two constituent components. Nonetheless, the two constituent components are generally supplied to the mixer by volumetric metering pumps which control the volumetric ratio of the two components, rather than their weights or masses. The volumetric ratio fails to account for any changes in density and changes in mass that may occur when the components are subjected to temperature or pressure change.
Also, production items often move along a production line at a set speed. Therefore, the flow rate of the dispensed composition should be kept or maintained as constant as possible so that the time required to dispense the proper amount of composition onto or into the production item remains constant.
An assembly line operation may further require that the composition be dispensed intermittently because the composition is applied to production items that are separated spatially and temporally. Dispensing compositions intermittently may cause a loss of flow control and/or ratio control. During the first few seconds of dispensing a composition, a transient imbalance phenomenon may arise from the elasticity of materials in the dispensing system and/or changing pressures caused by cycling the dispenser. When pressure changes, the volume of stored material between the mixer and the pump changes. In other words, changes in pressure may introduce an error into the weight or mass ratio of the constituent components because a higher pressure results in a component taking less volume than the component would otherwise take, or in an expansion or shrinkage of the hoses, fittings and tubes. The loss of control may result in inaccurately dispensed quantities or ratio of materials. This loss of flow control can occur separately or in addition to the loss of ratio control. A loss of ratio control occurs when the transient imbalance phenomenon causes the dispensing system to dispense too much or too little of one constituent material, thereby resulting in an improperly constituted end product. In other words, even if the ratio control is not lost during the first few seconds of dispensing a composition, the flow control may be lost. Therefore, it is desirous to control both the ratio of constituent materials and the flow rate of dispensing of the resulting composition.
Dispensing machines may be used to create various types of compositions. A dispensing machine may be required to dispense two or more constituent materials to form a first composition and then switch to dispense either the same constituent materials in a different ratio or other constituent materials to form a second composition. Thus, it is desirable for a dispensing machine to change what materials are dispensed, the quantities of materials dispensed and/or the ratio of constituent materials while maintaining the device's ability to control accurately the quantity, ratio, flow rate and other dispensing criteria. Current dispensing systems fail to satisfy these needs and require users to shut down the dispensing machine and go through a lengthy calibration cycle in order to adjust the machine to the viscosity and/or other properties of the constituent materials.
Some dispensing systems include vats capable of holding large amounts of a constituent material. Motor-driven agitators are placed inside the vat to maintain the material homogeneity. As the amount of material held in a vat is consumed, the agitator requires either less velocity or less current to mix the remaining material. However, in present dispensing systems, as the remaining material in a vat decreases, an agitator controlled by conventional means may over-agitate the material, resulting in frothing or the introduction of air bubbles. This frothing of the material could adversely affect the accuracy of the amount of material dispensed.
These concerns and problems may be further exacerbated when a dispensing system attempts to dispense a composition formed by mixing a solid powder with a liquid. Additional issues such as maintaining proper ratios and homogeneity arise.
Ideally, a dispensing system should be able to accurately control the ratio of each constituent material dispensed, the flow rate of each constituent material dispensed, the flow rate of the resulting composition, and the amount of the constituent materials and the resulting composition dispensed and be able to maintain such accuracy over time and various operating conditions. However, present dispensing systems fail to satisfy these attributes.